Linear motion guide units have been extensively applied in recent years between relatively sliding parts for linear reciprocating mechanism used in increased industrial fields including semiconductor fabricating equipment, machine tools, industrial robots, and so on. Most prior linear motion guide units have been ordinarily fed with lubricant every a preselected interval through any oiling port to ensure smooth lubrication for the rolling elements throughout a circulating circuit. Recently advanced machines and instruments, nevertheless, are increasingly needed to make them virtually maintenance-free from many aspects of conserving energy as well as keeping running cost and maintenance cost of equipment reasonably less than ever. Correspondingly, the linear motion guide units incorporated in the advanced machinery, especially the linear motion guide units of the sort of roller bearings are also challenged to keep down the consumption of lubricant to a minimum, along with realizing virtual maintenance-free operation where lubricant resupply is less needed over long-lasting operation. With the linear motion guide units constructed as stated earlier, continuous application of lubricant on or between the load-carrying race and the rolling element is indispensable to continue maintaining an adequate lubricant film around the rolling elements to keep them against metal-to-metal contact that might otherwise occur, thereby making sure of their high durability.
In Japanese Laid-Open Patent Application No. 2008-82,433 which is a commonly-assigned senior application, there is disclosed an example of the sliding system in which application of lubricant around the rolling elements was done at the turnaround passage to make the lubrication system simpler than ever in construction, along with maintenance-free for steady and positive oil lubrication. With the prior sliding system constructed as stated earlier, a porous compact impregnated with lubricant fits into a concavity made on an outward end surface of the end cap, which is in the face of the associated end seal. The concavity is deep pitted to open into the turnaround passage in the end cap. The porous compact impregnated with lubricant is placed in the concavity in the end cap in such a way as to expose in part itself to the turnaround passage through the deep pit. Thus, the porous compact makes contact with the rolling elements running through the turnaround passage to come to supply with the lubricant around the rolling elements.
Conventionally, the linear motion guide unit lubricated with oil impregnated in polymer member is known in the art and disclosed, for example in Japanese Laid-Open Patent Application No. H10-78 032. With the prior linear motion guide unit, an end cap is made therein with a curved passage to form a part of a circulatory passage which allows the rolling elements to run through there. A radially inward curved surface defining the curved passage is partially constituted with a return guide that is made of lubricant-containing polymer lined with a reinforcing member. The return guide fits into a groove semicircular in transverse section cut into the back of the end cap. The return guide conforming to the radially inward curved surface of the turnaround passage is made of potassium titanate whisker-reinforced polymer composites containing lubricant therein, which is integrally cast in the mold. With the return guide constructed as stated earlier, the reinforcing member accounts for a major percentage in content to increase the mechanical strength of the return guide and, correspondingly, the polymer to contain lubricant therein to be oozed for lubrication gets less in content, causing shortage of lubricant contained. Moreover, the polymer after the lubricant has oozed away comes to shrinkage or contraction to thwart the flow of lubricant inside the polymer. This would cause a shortage of worse supply of lubricant around the rolling elements.
Meanwhile, with the miniaturized linear motion guide unit in which tiny rollers are built as a four-row array around the guide rail having a width of for example 12 mm, our experts have ever tried doing the lubrication for the rollers inside the turnaround passage in the end cap using the same construction as disclosed in the commonly-assigned Japanese Laid-Open Patent Application No. 2008-82 433 recited earlier, in which the porous compact lying on the outward end surface of the end cap in the face of the associated end seal has a lubricant reservoir whose a part protrudes to be exposed in the turnaround passage through the opening pitted in the end cap. On the other side, the linear motion guide unit constructed as stated earlier normally has a binding strip to fasten a retainer plate to the slider, which is to keep the rolling elements against falling away from a load-carrying race, and for the sake of which the end cap on its outward end surface facing the end seal has grooves into which the bent ends of the binding strip fit to make snap-engagement with the associated end cap. Because the grooves to retain the binding strip take up a large space across the outward end surface of the end cap, it is impractical to make the porous compact large in volume at either side of the lubricant reservoir and the protrusion coming into contact with the rolling elements. It was found that making the porous compact excessively large in volume would render overall the linear motion guide unit worse or less in mechanical strength. Moreover, the protrusion of the porous compact, as undergoing repeated collisions with the rolling elements, is needed to have a mechanical strength enough to stand up to repeated impacts caused by the rolling elements.
Meanwhile, there are two members of the class of linear motion guide units, one of which has balls and the other has cylindrical rollers. With the linear motion guide unit in which cylindrical rollers or needles are selected as the rolling elements, alignment problems to guide rollers in good rolling order without getting leaned or tilted in rolling posture are more encountered, compared with the construction using balls as rolling elements. To cope with this, the cylindrical rollers have to be guided not only on their circular rolling surfaces, but also on their axially opposite end surfaces. Conventionally, there has been no linear motion guide unit constructed to meet with an aspect of allowing the rollers to roll through the circulating circuit over a long-lasting service life with adequate application of lubricant, making certain of steady reliability of the maintenance-free on lubricant application. Advanced technology has come to hope a linear motion guide unit which, even though simplified construction, is capable of retaining ample lubricant enough to conduct steady and sustainable lubrication for rollers, ensuring the smooth circulating motion of the rollers over a long-lasting period.